The simplest system names comets after the year in which they were observed (e.g. the Great Comet of 1680). Later a convention arose of using the names of people associated with the discovery (e.g. Comet Hale–Bopp) or the first detailed study (e.g. Halley's Comet) of each comet. During the twentieth century, improvements in technology and dedicated searches led to a massive increase in the number of comet discoveries, which led to the creation of a numeric designation scheme. The original scheme assigned codes in the order that comets passed perihelion (e.g. Comet 1970 II). This scheme operated until 1994, when continued increases in the numbers of comets found each year resulted in the creation of a new scheme. This system, which is still in operation, assigns a code based on the type of orbit and the date of discovery (e.g. C/2012 S1).

Before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year when they appeared e.g. the "Comet of 1702".

Possibly the earliest comet to be named after a person was Caesar's Comet in 44 BC, which was so named because it was observed shortly after the assassination of Julius Caesar and was interpreted as a sign of his deification. Later eponymous comets were named after the astronomer(s) who conducted detailed investigations on them, or later those who discovered the comet.

After Edmond Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Halley's Comet.[1] Similarly, the second and third known periodic comets, Encke's Comet[2] and Biela's Comet,[3] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition.

The first comet to be named after the person who discovered it, rather than the one who calculated its orbit, was Comet Faye – discovered by Hervé Faye in 1843. However, this convention did not become widespread until the early 20th century. It remains common today.

A comet can be named after up to three discoverers, either working together as a team or making independent discoveries (without knowledge of the other investigator's work). For example, Comet Swift–Tuttle was first found by Lewis Swift and then by Horace Parnell Tuttle a few days later; the discoveries were made independently and so both are honoured in the name.

In the past, when multiple comets were discovered by the same individual, group of individuals, or team, the comets' names were distinguished by adding a numeral to the discoverers' names (but only for periodic comets); thus Comets Shoemaker–Levy 1 – 9 (discovered by C. Shoemaker, E. Shoemaker & D. Levy). Today, the large numbers of comets discovered by some instruments makes this system impractical, and no attempt is made to ensure that each comet is given a unique name. Instead, the comets' systematic designations are used to avoid confusion.

Until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year (for example, Comet 1969i (Bennett) was the 9th comet discovered in 1969). Once the comet had been observed through perihelion and its orbit had been established, the comet was given a permanent designation of the year of its perihelion, followed by a Roman numeral indicating its order of perihelion passage in that year, so that Comet 1969i became Comet 1970 II (it was the second comet to pass perihelion in 1970)[4]

Increasing numbers of comet discoveries made this procedure awkward, as did the delay between discovery and perihelion passage before the permanent name could be assigned. As a result, in 1994 the International Astronomical Union approved a new naming system. Comets are now designated by the year of their discovery followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery (a system similar to that already used for asteroids), so that the fourth comet discovered in the second half of February 2006, for example, would be designated 2006 D4. Prefixes are also added to indicate the nature of the comet:

P/ indicates a periodic comet (defined for these purposes as any comet with an orbital period of less than 200 years or confirmed observations at more than one perihelion passage).[5]

C/ indicates a non-periodic comet (defined as any comet that is not periodic according to the preceding definition).

X/ indicates a comet for which no reliable orbit could be calculated (generally, historical comets).

D/ indicates a periodic comet that has disappeared, broken up, or been lost.[5]

A/ indicates an object that was mistakenly identified as a comet, but is actually a minor planet.

For example, Comet Hale–Bopp's designation is C/1995 O1. After their second observed perihelion passage, periodic comets are also assigned a number indicating the order of their discovery.[6] So Halley's Comet, the first comet identified as periodic, has the systematic designation 1P/1682 Q1. Comets that first received a minor planet designation keep the latter, which leads to some odd names such as P/2004 EW38 (Catalina–LINEAR).